This repository is a visual cheatsheet on the main topics in Backend-development. All the material is divided into topics and subtopics. The structure of the material consists of three parts:
-**Visual part** - various images/tables/cheatsheets for better understanding (may not be available). All pictures and tables are made from scratch, specifically for this repository.
-**Summary** - A very brief summary with a list of key terms and concepts. The terms are hyperlinked to the appropriate section on Wikipedia or a similar reference resource.
-**References to sources** - resources where you may find complete information on a particular issue. If possible, the most authoritative sources are indicated, or those that provide information in as simple and comprehensible language as possible.
> 🛠 The repository is under active development, so it is constantly updated and supplemented
* [Monolithic and microservice architecture](#monolithic-and-microservice-architecture)
* [Horizontal and vertical scaling](#horizontal-and-vertical-scaling)
</details>
[Additional and similar resources](#additional-and-similar-resources)
## Network & Internet
[Internet](https://en.wikipedia.org/wiki/Internet) is a worldwide system that connects computer networks from around the world into a single network for storing/transferring information. The Internet was originally developed for the military. But soon it began to be implemented in universities, and then it could be used by private companies, which began to organize networks of providers that provide Internet access services to ordinary citizens. By early 2020, the number of Internet users exceeded 4.5 billion.
Your computer has never been directly connected to the Internet. Because it can only see its local network to which other devices are connected via wired ([Ethernet](https://en.wikipedia.org/wiki/Ethernet)) or wirelessly (Wi-Fi, Bluetooth). To communicate with the Internet, you have a special minicomputer in your local network - [router](<https://en.wikipedia.org/wiki/Router_(computing)>). It then connects you to [Internet Service Provider](https://en.wikipedia.org/wiki/Internet_service_provider) which in turn connects to other higher-level providers. Thus, your message, transits through the network of several ISPs before reaching the destination network.
The Internet is just a long wire to which a small number of [Tier 1 providers](https://en.wikipedia.org/wiki/Tier_1_network) are directly connected. The ISPs below that are just renting access.
[Domain Names](https://en.wikipedia.org/wiki/Domain_name) are human-readable addresses of web servers available on the Internet. They consist of parts (levels) separated from each other by a dot. Each of these parts provides specific information about the domain name. For example country, service name, localization, etc.
- Who owns domain names
> [The ICANN Corporation](https://en.wikipedia.org/wiki/ICANN) is the founder of the distributed domain registration system. It gives accreditations to companies that want to sell domains. In this way a competitive domain market is formed.
- How to buy a domain name
> A domain name cannot be bought forever. It is leased for a certain period of time. It is better to buy domains from [accredited registrars](https://www.icann.org/en/accredited-registrars?filter-letter=a&sort-direction=asc&sort-param=name&page=1) (you can find them in almost any country).
<details>
<summary>🔗 <b>References</b></summary>
1. 📄 [**What is a Domain Name?** – MDN](https://developer.mozilla.org/en-US/docs/Learn/Common_questions/What_is_a_domain_name)
2. 📺 [**A Beginners Guide to How Domain Names Work!** – YouTube](https://youtu.be/Y4cRx19nhJk)
[IP address](https://en.wikipedia.org/wiki/IP_address) is a unique numeric address that is used to recognize a particular device on the network.
- Levels of visibility
> - External and publicly accessible IP address that belongs to your ISP and is used to access the Internet by hundreds of other users.
> - The IP address of your router in your ISP's local network, the same IP address from which you access the Internet.
> - The IP address of your computer in the local (home) network created by the router, to which you can connect your devices. Typically, it looks like 192.168.XXX.XXX.
> - The internal IP address of the computer, inaccessible from the outside and used only for communication between the running processes. It is the same for everyone - 127.0.0.1 or just _localhost_.
> One device (computer) can run many applications that use the network. In order to correctly recognize where and which data coming over the network should be delivered (to which of the applications) a special numerical number - a port is used. That is, each running process on a computer which uses a network connection has its own personal port.
- [IPv4](https://en.wikipedia.org/wiki/IPv4)
> Version 4 of the IP protocol. It was developed in 1981 and limits the address space to about 4.3 billion (2^32) possible unique addresses.
- [IPv6](https://en.wikipedia.org/wiki/IPv6)
> Over time, the allocation of address space began to happen at a much faster rate, forcing the creation of a new version of the IP protocol to store more addresses. IPv6 is capable of issuing 2^128 (is huge number) unique addresses.
[DNS (Domain Name System)](https://en.wikipedia.org/wiki/DNS) is a decentralized Internet address naming system that allows you to create human-readable alphabetic names (domain names) corresponding to the numeric [IP addresses](#ip-address) used by computers.
<details>
<summary>🔗 <b>References</b></summary>
1. 📄 [**What is DNS? Domain Name System explained** – freeCodeCamp](https://www.freecodecamp.org/news/what-is-dns/)
2. 📺 [**DNS (Domain Name System) explained. Types of Domain Name Servers** – YouTube](https://youtu.be/JkEYOt08-rU)
3. 📺 [**DNS as Fast As Possible** – YouTube](https://youtu.be/Rck3BALhI5c)
Modern [web applications](https://en.wikipedia.org/wiki/Web_application) consist of two parts: [Frontend and Backend](https://en.wikipedia.org/wiki/Frontend_and_backend). Thus implementing a [client-server model](https://en.wikipedia.org/wiki/Client%E2%80%93server_model).
The tasks of the Frontend are:
- Implementation of the user interface (appearance of the application)
> A special markup language [HTML](https://en.wikipedia.org/wiki/HTML) is used to create web pages. <br> > [CSS](https://en.wikipedia.org/wiki/CSS) style language is used to style fonts, layout of content, etc. <br> > [JavaScript](https://en.wikipedia.org/wiki/JavaScript) programming language is used to add dynamics and interactivity. <br>
> As a rule, these tools are rarely used in their pure form, as so-called [frameworks](https://2020.stateofjs.com/en-US/technologies/front-end-frameworks/) and [preprocessors](https://www.freecodecamp.org/news/css-preprocessors/) exist for more convenient and faster development. <br>
- Creating functionality for generating requests to the server
> These are usually different types of input forms that can be conveniently interacted with.
- Receives data from the server and then processes it for output to the client
Tasks of the Backend:
- Handling client requests
> Checking for permissions and access, all sorts of validations, etc.
- Implementing business logic
> A wide range of tasks can be implied here: working with databases, information processing, computation, etc. This is, so to speak, the heart of the Backend world. This is where all the important and interesting stuff happens.
- Generating a response and sending it to the client
[Browser](https://en.wikipedia.org/wiki/Web_browser) is a client which can be used to send requests to a server for files which can then be used to render web pages. In simple terms, a browser can be thought of as a program for viewing HTML files, which can also search for and download them from the Internet.
The use of VPNs and Proxy is quite common in recent years. With the help of these technologies, users can get basic anonymity when surfing the web, as well as bypass various regional blockages.
> A technology that allows you to become a member of a private network (similar to your local network), where requests from all participants go through a single public IP address. This allows you to blend in with the general mass of requests from other participants. <br>
>
> - Simple procedure for connection and use. <br>
> - Reliable traffic encryption. <br>
> - There is no guarantee of 100% anonymity, because the owner of the network knows the IP-addresses of all participants. <br>
> - VPNs are useless for dealing with multi-accounts and some programs because all accounts operating from the same VPN are easily detected and blocked. <br>
> - Free VPNs tend to be heavily loaded, resulting in unstable performance and slow download speeds. <br>
> A proxy is a special server on the network that acts as an intermediary between you and the destination server you intend to reach. When you are connected to a proxy server all your requests will be performed on behalf of that server, that is, your IP address and location will be substituted. <br>
>
> - The ability to use an individual IP address, which allows you to work with multi-accounts. <br>
> - Stability of the connection due to the absence of high loads. <br>
> - Connection via proxy is provided in the operating system and browser, so no additional software is required. <br>
> - There are proxy varieties that provide a high level of anonymity. <br>
> - The unreliability of free solutions, because the proxy server can see and control everything you do on the Internet. <br>
[Hosting](https://en.wikipedia.org/wiki/Web_hosting_service) is a special [service provided](https://en.wikipedia.org/wiki/Internet_hosting_service) by hosting providers, which allows you to rent space on a server (which is connected to the Internet around the clock), where your data and files can be stored. There are different options for hosting, where you can use not only the disk space of the server, but also the CPU power to run your network applications.
> At this level, bits (ones/zeros) are encoded into physical signals (current, light, radio waves) and transmitted further by wire ([Ethernet](https://en.wikipedia.org/wiki/Ethernet)) or wirelessly ([Wi-Fi](https://en.wikipedia.org/wiki/Wi-Fi)).
- [Data link layer](https://en.wikipedia.org/wiki/Data_link_layer)
> Physical signals from layer 1 are decoded back into ones and zeros, errors and defects are corrected, and the sender and receiver [MAC addresses](https://en.wikipedia.org/wiki/MAC_address) are extracted.
> The layer responsible for data transfer. There are two important protocols: <br>
>
> - [TCP](https://en.wikipedia.org/wiki/Transmission_Control_Protocol) is a protocol that ensures reliable data transmission. TCP guarantees data delivery and preserves the order of the messages. This has an impact on the transmission speed. This protocol is used where data loss is unacceptable, such as when sending mail or loading web pages. <br>
> - [UDP](https://en.wikipedia.org/wiki/User_Datagram_Protocol) is a simple protocol with fast data transfer. It does not use mechanisms to guarantee the delivery and ordering of data. It is used e.g. in online games where partial packet loss is not crucial, but the speed of data transfer is much more important. Also, requests to DNS servers are made through UDP protocol.
> Responsible for opening and closing communications (sessions) between two devices. Ensures that the session stays open long enough to transfer all necessary data, and then closes quickly to avoid wasting resources.
> Transmission, encryption/decryption and data compression. This is where data that comes in the form of zeros and ones are converted into desired formats (PNG, MP3, PDF, etc.)
[HTTP (HyperText Transport Protocol)](https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol) is the most important protocol on the Internet. It is used to transfer data of any format. The protocol itself works according to a simple principle: request -> response.
- [Structure of HTTP messages](https://developer.mozilla.org/en-US/docs/Web/HTTP/Messages)
> [GET](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/GET) - data retrieval request <br> [POST](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST) - request with data to create a new record <br> [PUT](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/PUT) - request with data to change existing record <br> [DELETE](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/DELETE) - deletion request <br> Others: [HEAD](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/HEAD), [CONNECT](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/CONNECT), [OPTIONS](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/OPTIONS), [TRACE](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/TRACE), [PATCH](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/PATCH). <br>
- [Response status codes](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status)
> Each response from the server has a special numeric code that characterizes the state of the sent request. These codes are divided into 5 main classes:
> Because the HTTP protocol does not allow you to save any information about the status of previous requests/responses, you need to use cookies. Cookies allow the server to store various information on the client side, which the client can then send back to the server. In particular, cookies can be used for authorization or to save various settings/configurations.
> A special header that allows you to recognize and eliminate certain types of web application vulnerabilities.
- [HTTP/1.0 vs HTTP/1.1 vs HTTP/2](https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol#Persistent_connections)
> The main innovation in version 1.1 is the permanent connection mode, which allows you to send several requests per connection. In version 2, the protocol became binary, with the ability to transmit data from multiple streams on the same channel.
Compared to the [OSI model](https://github.com/cheatsnake/backend-cheats/blob/master/README_ENG.md#osi-network-model), the [TCP/IP](https://en.wikipedia.org/wiki/Internet_protocol_suite) stack has a simpler architecture. It is widely used and was first used as the basis for the creation of a global network, and then to describe the workings of the Internet.
1. 📄 [**What is the TCP/IP Model? Layers and Protocols Explained** – freeCodeCamp](https://www.freecodecamp.org/news/what-is-tcp-ip-layers-and-protocols-explained/)
2. 📺 [**What is TCP/IP?** – YouTube](https://youtu.be/PpsEaqJV_A0)
The quality of networks, much less the Internet, is far from ideal. This is due to the complex and dispersed network structure in different devices. Therefore, on the functioning of the network affects a huge number of factors. For example: the stability of the connection between the client device and its router, the quality of service of the provider, the power and performance of the server, the physical distance between the client and the server, etc.
> Not all packets traveling over the network can reach their destination. This happens most often when using wireless networks or due to [network congestion](https://en.wikipedia.org/wiki/Network_congestion).
- [Round Trip Time (RTT)](https://en.wikipedia.org/wiki/Round-trip_delay)
> The time it takes for the data packet to reach its destination + the time to respond that the packet was received successfully.
> A procedure that allows you to trace to which nodes, with which IP addresses, a packet you send before it reaches its destination. Tracing can be used to identify computer network related problems and to examine/analyze the network.
> The most important PC component to which all other elements are connected.
>
> - [Chipset](https://en.wikipedia.org/wiki/Chipset) - set of electronic components that responsible for the communication of all motherboard components.
> - [CPU socket](https://en.wikipedia.org/wiki/CPU_socket) - socket for mounting the processor.
> - [VRM (Voltage Regulator Module)](https://en.wikipedia.org/wiki/Voltage_regulator_module) – module that converts the incoming voltage (usually 12V) to a lower voltage to run the processor, integrated graphics, memory, etc.
> - Slots for RAM.
> - Expansion slots [PCI-Express](https://en.wikipedia.org/wiki/PCI_Express) - designed for connection of video cards, external network/sound cards.
> - Slots [М.2](https://en.wikipedia.org/wiki/M.2) / [SATA](https://en.wikipedia.org/wiki/SATA) - designed to connect hard disks and SSDs.
> The most important device that executes instructions (programme code). Processors only work with 1 and 0, so all programmes are ultimately a set of binary code.
>
> - [Registers](https://en.wikipedia.org/wiki/Processor_register) - the fastest memory in a PC, has an extremely small capacity, is built into the processor and is designed to temporarily store the data being processed.
> - [Cache](https://en.wikipedia.org/wiki/CPU_cache) - slightly less fast memory, which is also built into the processor and is used to store a copy of data from frequently used cells in the main memory.
> - Processors can have different [architectures](https://en.wikipedia.org/wiki/Processor_design). Currently, the most common are the [x86](https://en.wikipedia.org/wiki/X86-64) architecture (desktop and laptop computers) and [ARM](https://en.wikipedia.org/wiki/ARM_architecture_family) (mobile devices as well as the latest Apple computers).
> Fast, low capacity memory (4-16GB) designed to temporarily store program code, as well as input, output and intermediate data processed by the processor.
> A separate card that translates and processes data into images for display on a monitor. This device is also called a discrete graphics card. Usually needed for those who do 3D modelling or play games. <br> > [Built-in graphics card](https://en.wikipedia.org/wiki/Graphics_processing_unit#Integrated_graphics_processing_unit) is a graphics card built into the processor. It is suitable for daily work.
[Operating system (OS)](https://en.wikipedia.org/wiki/Operating_system) is a comprehensive software system designed to manage a computer's resources. With operating systems, people do not have to deal directly with the processor, RAM or other parts of the PC.
OS can be thought of as an abstraction layer that manages the hardware of a computer, thereby providing a simple and convenient environment for user software to run.
- Main features
> - RAM management (space allocation for individual programms)
> - Loading programms into RAM and their execution
> - Execution of requests from user's programms (inputting and outputting data, starting and stopping other programms, freeing up memory or allocating additional memory, etc.)
> - Interaction with input and output devices (mouse, keyboard, monitor, etc.)
> - Interaction with storage media (HDDs and SSDs)
> - Providing a user's interface (console shell or graphical interface)
> - Logging of software errors (saving logs)
- Additional functions (may not be available in all OSs)
> - Organise [multitasking](https://en.wikipedia.org/wiki/Computer_multitasking) (simultaneous execution of several programms)
> - Delimiting access to resources for each process
> The central part of the operating system which is used most intensively. The kernel is constantly in memory, while other parts of the OS are loaded into and unloaded from memory as needed.
> The system software that prepares the environment for the OS to run (puts the hardware in the right state, prepares the memory, loads the OS kernel there and transfers control to it (the kernel).
1. 📄 [**What is an OS? Operating System Definition for Beginners** – freeCodeCamp](https://www.freecodecamp.org/news/what-is-an-os-operating-system-definition-for-beginners/)
2. 📄 [**Windows vs MacOS vs Linux – Operating System Handbook** – freeCodeCamp](https://www.freecodecamp.org/news/an-introduction-to-operating-systems/)
> A kind of container in which all the resources needed to run a program are stored. As a rule, the process consists of:
>
> - Executable program code <br>
> - Input and output data <br>
> - [Call stack](https://en.wikipedia.org/wiki/Call_stack) (order of instructions for execution) <br>
> - [Heap](https://en.wikipedia.org/wiki/Memory_management#Manual_memory_management) (a structure for storing intermediate data created during the process) <br>
> An entity in which sequences of program actions (procedures) are executed. Threads are within a process and use the same address space. There can be multiple threads in a single process, allowing multiple tasks to be performed. These tasks, thanks to threads, can exchange data, use shared data or the results of other tasks.
> The ability to perform multiple tasks, but using a single processor core. This is achieved by dividing tasks into separate blocks of commands which are executed in turn, but switching between these blocks is so fast that for users it seems as if these processes are running simultaneously.
1. 📄 [**Concurrency, parallelism, and the many threads of Santa Claus** – freeCodeCamp](https://www.freecodecamp.org/news/concurrency-parallelism-and-the-many-threads-of-santa-claus/)
2. 📺 [**Concurrency vs Parallelism** – YouTube](https://youtu.be/Y1pgpn2gOSg)
3. 📺 [**Concurrency is not Parallelism by Rob Pike** – YouTube](https://youtu.be/oV9rvDllKEg)
A mechanism which allows to exchange data between threads of one or different processes. Processes can be run on the same computer or on different computers connected by a network. [Inter-process communication](https://en.wikipedia.org/wiki/Inter-process_communication) can be done in different ways.
> The easiest way to exchange data. One process writes data to a certain file, another process reads the same file and thus receives data from the first process.
> In particular, IP addresses and ports are used to communicate between computers using the TCP/IP protocol stack. This pair defines a socket (_socket_ corresponding to the address and port).
